Voltage amplifying thermionic arrangement



vJuly 3, 1934. o. EMERSLEBEN VOLTAGE AMPLIFYING THERMIONIC ARRANGEMENT Filed sept. 28, 1929 Patented July 3, 1934 VOLTAGE AMPLIFYING THERMIQNIG ARRANGEMENT Otto Emersleben, Berlin, Germany Application September 28, 1929, Serial No. 395,767 In Germany September 28, 1928 5 Claims.

My invention relates to improvements in so called thermionic amplifiers. One feature of my invention concerns resistance coupled amplifiers operating as voltage amplifiers with a special view to dimension the elements of the connections.

My invention relates furthermore to a certain type of amplifying tubes or a thermionic amplifying system which will preferably be used in such connections.

My invention concerns also certain arrangements for obtaining the grid bias in a low frequency amplifier.

Further arrangements of my invention may be seen from the following specification.

Before describing my invention I wish to explain some technical terms which are not always used in the same manner.

In a thermionic amplifying tube, more particularly in a three-electrode-tube, containing in a vacuum a heated filament operating as cathode, a grid operating as controlling electrode anda plate operating as anode, an amplifying effect is in a known manner generally obtained by the fact Jthat in operation the voltage variations existing between the controlling electrode and the filament cause greater Voltage variations between the plate and the filament. The ratio (V) of the voltage variations of the output circuit (plate circuit) to the voltage variations of the input circuit (grid circuit) of such a tube shall becalled amplification or degree of amplification. This ratio (V) therefore depends not only -on the tube but to a considerable part on the connection of coupling elements in which such a tube is operating. In all well operating amplifiers however this degree of amplification V must be greater than 1, atleast for the frequencies for which the amplifier has to be used.

The amplification depends considerably upon the qualities of the tube, more particularly upon theamplication factor (y) of the tube. When using several tubes connected in cascade the degree of amplification V of every stage of the arrangement will always be smaller than the amplification factor g.

The amplification factor may bedened electrostatically to be a ratio of two inner capacities of the electrodes of a tube. With a well evacuated tube having no electrical space kcharge between grid and plate, the amplification Afactor g is equal to the ratio of the capacity between grid and filament on one side, to thecapacitybetween plate and filament on the other side. The amplification factor-therefore `depends on the structure of the tube only, and not on its connections.

My invention indicates how the amplification factor g of a tube must be taken in order to give in a certain connection a degree of amplification V as high as possiblei According to former publications there was existing the opinion that in order to obtain a great degree of amplification V it is only necessary to take tubes having a very high amplification factor g. This opinion seems to have been favoured by the fact that both terms are similar. I have found however, that this is a technical error. By means of theoretical calculations which have later on been verified by means of experiments I have found that in an amplifying arrangement, more particularly in a resistance coupled amplifier having a certain cathode, certain coupling elements and certain voltages of the anode-current-source, there is existing a well defined amplication factor y0 by means of which la maximum V0 of the degree of amplication may be obtained. Therefore not only in replacing the tube with the amplification factor g by another one having a smaller amplification factor g but also in replacing the tubeV with the amplification factor go by a tube with a greater amplification factor g the amplification V will become smaller than the said best value V0. This best amplification V0 however and even the best amplification factor go depends upon the values of the coupling elements and the anode and grid voltages used in said connections.

The development of the last years, due 'to the experiments made Aby Manfred von Ardenne, had the result that in resistance coupled amplifiers anode resistances are usedl having resistances of some megohms and amplifying tubes are used having amplification factors of 30-40. It was not possible by means of these tubes to get a degree of amplification V which was greater than a limited value smaller than the amplification factor g. As the amplification factor g is the asymptotical upper vlimit for the degree of amplification V, an amplification of nearly one `third of the amplification factor has been obtained. Therefore I do not :know any resistance coupled low-frequency amplifier which `has obtained in a pure cascade arrangement, Without using any reaction, a considerably greater amplification than 12 per stage. v

If in some descriptions resistance coupled amplifiers with greater amplications than l2 have beenrspecified, the Word amplification has been usedinanothermeaning. Formerly the amplification was not indicated by the degree of voltage amplification as 'l have doneA it but by the ratio of the energies of the output and the input circuit. Forinstance in theftish patent specification No. 129,313 on page 3, line 'Leven in a high frequency amplifying arrangement using resistance coupled normal Lthree-electrocle-tubes the amplification has loeencalledY 100 and this is supposed t0 be theamplifaiiollof the energie The-Voltage ,@mpllcftqn .,frrespondng With thisaaplinaaon offre energy wmhave been no fio i' so more than the square root of 100, that is to say, no more than 10. f y

By means of my invention'however, it is possible to get by means of one amplification stage a considerably greater amplification up to 100 and more. Therefore for purposes for which until now a triple valve has been used when using my invention only a double valve will be necessary having one voltage amplifying stage and the output stage.

According to my invention I use tubes having a great amplification factor of more than or better than 100 or even of the order of 1000 or more. I have found that it may be very useful to take normal tubes with one grid having amplification factors of 100 and more, more particularly in resistance amplifying connections. If however the amplification factor has to be greater than 1000 it may be useful to take tubes having several grids because in such a way a great amplification factor may be obtained with most simple means.

From the following explanation it may be seen why formerly these tubes have not been used.

It is a well known fact that the amplication factor is only an upper limit for the degree of amplification which however generally will not be reached. With tubes having an amplification factor of 9:10 the amplification generally will be nearly 6 or 8 (dependent on the connections, the

resistances etc.). The degree of amplification therefore is obtained from the amplication factor by multiplying the amplification factor g with the quotient V.g=m, a number smaller than 1, which shall be called the reducing factor. When replacing in the same connection one tube by another one having a greater amplification factor, the reducing factor m has been found to become smaller in such a way that the product m.g=V, that is to say the amplification, does not increase in these cases as rapidly as the amplification factor does. In many cases even if 9:30 or g=40 the amplification was found to give no further increase. vThis may have been the reason why until now as far as I am informed, g has not been taken larger.

It has been recognized, that the absolute degree of the amplification may be more raised the larger the amplication factor of the tube is chosen. But practically hereby a limitation is reached, caused by certain practical factors. It has been found, that together with the enlarging of the amplification factor of the tube, it is necessary to raise also the voltage of the anode-current-source, and the coupling-elements have to possess such values, that with regard to the necessary high isolations and the hereby diminished range of the controlling input-voltage respectively the otherwise occurring distortion caused by frequency dependency, it has practically no value K to surpass an amplification factor of about 25%. It is important, that up to now the amplifiers are so dimensioned for fulfilling said certain practical demands, i. e. for choosing not too enormous voltages of the anode-current-voltage and a certain frequency-independency in a large controlling range. Certain amplification factors and lcoupling elements are a result of this demand, having hereby intentionally renounced the result of a maximal degree of the amplification. In these well known arrangements therefore higher amplifications could have been reached by a difrent choice of the constants.

1 havev found however, that it isl not only necessary to take tubes having a great amplification factor but that the amplification factor has to be chosen with reference to the connections in which the tube is to be used. I have found that the optimal amplification factor of the tube depends upon the voltage of the plate battery, on the plate resistances, on the electron discharging qualities of the filament emission and on the potential existing between the filament and the controlling electrode (grid bias).

If in a resistance coupled amplifier a high ohmic plate resistance of some megohms is used, a considerable drop of the voltage of the plate battery will be caused by the plate resistance itself. By taking a greater plate voltage, whereas all other elements of the connection may be kept constant, the reducing factor m and therefore the amplification V will become greater. But for the same reason there will be produced a greater plate current and therefore a greater drop of potential in the plate resistance. Therefore the plate current voltage will not increase in the same way as the plate does.

In former publications rules have been given to take a certain amplification factor. Theserules generally were given to a certain plate voltage between cathode and anode of the tube. This voltage, however, for amplifiers having a high plate resistance is indefinite by the variable slope of voltage in the plate resistance and therefore not able to give a good idea for the amplification factor which was to be chosen with respect to the definite voltage of the plate battery being used.

Therefore in this way it was until now not possible to get those connections which give a really good amplifier.

I have found however, that for indicating the amplification factor of the tube that must be used in order to obtain the greatest degree of amplification in calculations as well as in experiments the exact voltage of the plate battery or any other source of plate voltage may be taken. Then, a certain amplification factor may be found which will give the optimal amplification when the plate resistance and the qualities of the filament are kept constant. Doing so I suppose that in using several tubes having different amplification factors the distance from the filament to the grid will not be changed. Therefore this distance may have the least value that may be taken without causing a danger of short-circuit from the grid to the filament.

The amplification depends furthermore to the certain degree on the grid bias. Until now generally for the purpose of low-frequency amplifying arrangements a certain negative grid bias was taken, which was generally not less than the voltage resulting from one stage of an element of an electrolytic grid bias battery, this voltage being of the order of 1.4 to 1.6 volts.

I have found that in using tubes with great amplification factors it is important for obtaining a great degree of amplification to take the grid bias smaller than it has been taken until now. According to my invention the grid bias should be taken as small as possible. It will be Vuseful to use a potentiometer in order to get 'tion factors to take no grid bias at all.

If the plate battery has a certain voltage Eb, a tube having an amplification factor y should be used with a negative grid bias, if any, less than In connections in which only small variations of potential are acting on the controlling electrode, more particularly in the input side of the first stage of a low-frequency amplifying arrangement, these variations will not be great enough to. cause a great negative grid bias to be taken. In such stages only in which the grid is controlled by potential variationsof the order of 1 volt, it will nevertheless be necessary to take a certain negative grid bias. In many cases these further stages are not necessary when using my invention, because one stage will already be able to give the sufficiently great amplification.

If, however, it will be necessary to use a further voltage amplifying stage the amplification factor go must be taken with respect to this grid bias. I have found that, generally speaking, when using a negative grid bias, the amplification factor y0 must be taken smaller than when using no grid bias at al1.

My invention may be best understood by describing it with respect to the accompanying drawing.

In the drawing Fig. l shows by means of example the connections of a single stage of a resistance coupled amplifier.

Fig. 2 shows the connections of a multistage receiver in which the low frequency amplifying stages are constructed according to my invention.

Fig. 3 shows in a somewhat more detailed view how an amplifying system for a low frequency amplifying stage according to Fig. 2 may be constructed and Fig. 4 shows some characteristic lines of an amplifying system according to Fig. 3.

In Fig. 1 the electron discharge tube 1 consists of an anode 2, a grid 3 and an equipotential cathode 4 which is heated from the filament 5 by means of the heating-battery 6. The electrodes of the tube 1 may be arranged with respect to one another in such a Way that the amplification factor is great, say greater than 100. My inven-V` tion is not restricted to the use of an equipotential cathode. It is however, not useful to provide any means in such a way that no considerable drop of voltage is caused along the cathode by the heating current. This can be done by using a bifilar current supply or by taking such an equipotential cathode. In such a way I will obtain well defined differences of potential between the electrodes of the amplifying system. This is in an electrode system having a great amplification factor, and is a good way to get best results.

In Fig. 1 in the anode circuit of the tube 1 an anode current source 7 having a voltage Eb i-s 1 connected to the cathode 4 and between this an ode battery 7 and the anode 2 the high ohr-nic anode resistance 8 is connected. The value of the resistance 8 may be R (of the order of one megohm) The control-grid 3 may be connected to a grid bias 9 having a non-positive voltage Eg or if no grid bias is used with a voltage O with respect to the cathode 4. Between the terminals 10 of the grid and l1 of the grid bias 9 or of the ca-t-hode 4 is carried the input control voltage which is to be amplified. This input control voltage may be obtained for instance from electric resistances connected to any preceding receiving arrangement in aerial or to a. preceding stage of a multistage amplifying cascade.

may be the drop of potential caused in the anode resistance 8 by means of the anode current Ja if the points 1'0 'and 11 Iof the input side are directly connected to one another, that `is to say if there If this quotient depends on the value of eg, perhaps if an audion effect is acting, the degree of amplification will be equal to the limit Value of this quotient for eg=0.

If g is the amplification factor of the system 1 which may be called the controlling voltage.

In the case Where no grid current is fiowing the anode-current will essentially depend upon the controlling voltage. For small values of this voltage it may be proportional to Ecff up to Eef.

I may suppose for the purpose of an example that the anode current is proportional to the square of the controlling voltage Ec, that is to say that The factor c indicates how the cathode is emitting electrons. Y

The plate voltage Ea, that is the difference `of potential between cathode 4 and the plate 2, is not equal to the voltage Eb of the plate battery. The difference between this is a considerable one if R is of high ohmic value. The exact value of this difference however, can not exactly be cited because it depends on the plate current and this plate current, on its side, depends on the plate voltage. In this way the conditions are complicated in such a way that they seem to have not been well understood before.

I have found however that certain Values of the amplification factor give a maximum degree of amplification.

Let is assume that Eg=0, that is to say that the battery 9 is not existing. In this case I will generally have a greater degree of amplification V than if a certain grid bias is used.` For Eg=0 I have calculated the maximum of Vif Eb, R `and c have `certain given values. This maximum is obtained if 12X/1 L one e scREfw/cREb that is to say if the amplication factor has nearly the value In this case the maximum degree of amplification V to be obtained is Therefore in this case the reducing factor will be m=0.38.

'Ihis optimum y@ of the amplification factor g causing a maximum of the amplification V changes if the grid bias Eg is used. Furthermore it changes if the characteristic is not exactly that of the square. I vhave found, however, that in a certain interval of the grid bias great variations of the maximum of the amplification V0 are not obtained. I have found therefore that it will be useful, even if the characteristic of the cathode is another one and if one without different grid bias is used, to take the amplification factor of the amplifying system 1 in such a way that it is lying within a certain interval which contains the above indicated optimum go. It will be sufficient if the amplification is greater or equal to a/CRE but it is not greater than five fold value, that is to say, if

ia/oREbgvcREb A value c which corresponds to a certain voltage amplifying system of to-day is l milliampere Y so voie If such a cathode is used with the plate resistance R=3.2 megolims and a plate battery with the voltage of 256 volts an amplifying system of the amplification factor 170:200 will be found useful as may be calculated from the above indicated formula.

If, however, only a plate battery of 64 volts were used it would be necessary to take a system with an amplification factor 90:100. In the first case the best amplification that may be obtained would be 72, in the second case this would be 32, if all losses caused by the frequency of potential variations are neglected. If, however, different voltages are given to the plate battery there are existing two possibilities of diniensioning the amplifier. If it is desired to have a very great amplification then the amplification factor must be taken in such a way that it is the optimal amplification factor for the greatest of the voltages of the plate battery. In this case, when the plate battery is 256 an amplification of 'I8 may be obtained by means of a tube having an amplification factor of 100. If, however, the plate battery has only 64 volts with the same tube only an amplification of V=33 will be obtained. In this case, in the formula, go is taken according to the greater value of the plate voltage. The best amplification will be obtained for this voltage, a smaller amplification however for smaller plate voltages.

If however I wish to have small variations of the amplification even if considerable variations of the voltage occur, then the amplification factor should be taken in such a way that it is an optimum for the smallest value of the plate resistance` Referring to the above indicated example: if the'variations of the voltage occur between 64 and 256 volts with a tube having an amplification factor of g0=100, for small plate battery with a voltage of 64 volts I would have the optimum degree of amplification 38. For the greater plate voltage of 256 volts I would have a greater amplification of 64 but the greatest possible which has been calculated would be equal to 78.

.- If in describing my invention I have used the words plate battery, this was done also in such cases if the plate voltage for the amplifier may be taken from any other source e. g. from the mains. If I have usedv the words thermionic amplifier I wish to speak of any amplifying arrangement in which a discharging of electrons is controlled by a certain electrode even if cold cathodes are used, such as in photoelectric arrangements, which have the advantage that their cathodes are at a constant potential.

Fig. 2 shows the connection of the receiver in which my invention may be used. Anaerial 14 may be coupled to an oscillatory circuit 15 connected to a two stage resistance coupled high frequency amplifier 16 in which the plate resistances 1'7 and 18 may have about 20,000 ohms and the coupling condensers 19 and 20 may have 100 cm. The last coupling condenser 20 is connected by means of a grid leak resistance 21 of say 20,000 ohms to a two stage low frequency double valve 22. The first stage of this double valve is constructed according to my invention whereas the second stage is operating as a normal power amplifying and stage 23 for the purpose of Voperating the loudspeaker 24. Both stages in the double valve 22 are connected by means of a plate resistance 8 with R=4 or 5 megohms and a coupling condenser 25 of 200 cm. The grid leak resistance 26 may have 8 megohms. In such an amplifying system in which the amplification factor of the first stage is g=150 there has been reached the best results. It is shown in Fig. 3 in more detail. The plate has a cylindrical shape, the diameter of which is about 15 mm. and the length 18 mm. Each grid has a diameter of about with which good results have been obtained has characteristic lines as shown in Fig. 4. Fig. 4 shows for a plate battery Eb of 150 and 200 volts how the plate current varies with the grid voltage. Fig. 4 shows the so called working characteristics measured with a plate resistance of 4 megohms. The plate resistance causes for different values of the plate current different drops of voltage. Therefore to such a characteristic line different values of the plate voltage En would correspond. Having the normally known static characteristics of the plate voltages from which directly the constant c of the discharging current may be calculated a certain reduction from the plate battery voltage Eb to the plate Voltage Ea must be made.

However, even without 4suoli a reduction Fig. 4 shows nearly the amplification factor of the tube of Fig. 3. In Fig. 4 for the plate current of 0.01 niilliampere this same current will be caused for a plate battery of 150 volts on one side and of 200 volts-on the other side by grid voltages which differ from one another about nearly 1/3 volt. The amplification factor of the valve is nearly equal to this difference of the plate voltages (20G-150:50 volts) divided by the difference of the grid voltages which cause the same platecurrent Ja-that is to say g=:1/3=150.

When using an amplifier system according to Figs. 3 and 4 I have gotten good results with a grid bias of 0.6 volt.

Generally in using my invention the grid bias has to be taken as small as possible. If the Variations of the grid voltage which are to be amplified are very small it will be possible to use no grid bias Vat all but even if this is not possible it will be useful to take a grid voltage which is smaller than the voltage of the normal electrolytic element (nearly 1.5 volt). For this purpose, in Fig. 2, such a battery 9 is connected by means of a potentiometer in such a way that voltages between 0 and 1.5 volt may be connected to the grid 3 of the amplifying system.

My invention is not restricted to the examples of Figs. 3 and 4. I have found that very good results have been obtained with still greater amplification factors than 1000 if only Very small variations had to be amplified. In such cases an equipotential cathode 4 may be combined with a bifilar lead filament 5 as indicated in Fig. 2. In this case even the filament itself will cause no electrical field.

My invention is not restricted to a certain interval of frequencies of the variations which are amplified. The above indicated examples are concerning perhaps those cases in which only very small capacitive influences are existing. This will depend on the connection and on the frequencies. My invention will give Very good results for amplifying low frequencies more particularly audio frequencies, but it may be used also for higher frequencies if connections are taken in which the capacitive influences are sufficiently small.

It is a known fact that by arranging several amplifying systems in the same vacuum of a multiple valve these disagreeable capacities are relatively smaller; nevertheless because of these capacities generally the amplification will be smaller than V=0.38.g. If however, the ampliu cation go is taken in the above indicated interval of l to 5 the influences may be relatively small for frequencies which are not too great.

I claim:

1. In an amplifying arrangement several tubes coupled together' by means of resistance coupling elements, in said coupling elements anode resistances of high ohmic resistance in the order of 1 megohm and more, said resistances being connected with an anode-current source, said tubes having an amplification factor of more than dsa/CRE., but not more than asa/Wb R being the resistance of said anode coupling resistance, Eb the voltage of said anode-current source and c the characteristic constant of emission of the filaments of said tubes, whereby the anode-current-grid-Voltage characteristic is correctly or approximately represented by the emission formula JazcEstz, Ja being the anode-current and Est the effective controlling voltage of the grid.

2. In an amplifying arrangement several tubes coupled together by means of resistance coupling elements, in said coupling elements anode resistances of high ohmic resistance in the order of 1 megohm and more, said resistances being connected with an anode-current source, said tubes having an amplification factor approximately equal to R being the resistance of said anode coupling resistance, Eb the voltage of said anode-current source and c the characteristic constant of emission of the filaments of said tubes, whereby the anode-current-grid-voltage characteristic is correctly or approximately represented by the emission formula Ja=cEsi2, Ja being the anode-current and Est the effective controlling voltage of the grid.

3. In an amplifying arrangement several tubes coupled together by means of resistance coupling elements, in said coupling elements anode resistances of high ohmic resistanceV in the order 0f 1 megohm and more, said resistances being connected with an anode-current source, said tubes having an amplification factor of more than dsx/CREI, but not more than 2.51/ CREb R being the resistance of said anode coupling resistance, Eb the voltage of said anode-current source and c the characteristic constant of emission of the filaments of said tubes, whereby the anode-current-grid-voltages characteristic is correctly or approximately represented by the emission formula JazcEsiZ, Ja being the anodecurrent and Esi the effective controlling voltage of the grid, said tubes having an amplification factor of more than 1000.

4. In an amplifying arrangement several tubes coupled together by means of resistance coupling elements, in said coupling elements anode resistances of high ohmic resistance in the order of 1 megohm and more, said resistances being connected with an anode-current source, said tubes having an amplification factor of more than osa/CRE., but not more than asa/CRE,

R being the resistance of said anode coupling resistance, Eb the voltage of said anode-current source and c the characteristic constant of emission of the filaments of said tubes, whereby the anode-current-grid-voltage characteristic is correctly or approximately represented by the emission formula Ja=cEsi2, Je being the anode-current and Esi the eective ctonrolling voltage of the grid, said tubes having several grids and an amplification factor of more than 1000.

5. In an amplifying arrangement several tubes coupled together by means of resistance coupling elements, in said coupling elements anode resistances of high ohmic resistance in the order of 1 megohrn and more, said resistances being connected with an anode-current source, said tubes having an amplification factor of more than dsa/ORE, but not more than .asa/CRE,

R being the resistance of said anode coupling resistance, Eb the voltage of said anode-current source and c the characteristic constant of emission of the filaments of said tubes, whereby the anode-current-grid-voltage characteristic is correctly or approximately represented by the emission formula JazcEstZ, Ja being the anode-current and Est the effective controlling voltage of the grid, a grid bias of less than 1.5 volts for said tubes.

OTTO EMERSLEBEN.

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